High flexural strength ceramic fiber reinforced silicon carboxide composite

ABSTRACT

An improved fiber reinforced glass composite includes a carbon-coated refractory fiber in a matrix of a black glass ceramic having the empirical formula SiCxOy where x ranges from about 0.5 to about 2.0, preferably 0.9 to 1.6 and y ranges from about 0.5 to 3.0, preferably 0.7 to 1.8. Preferably the black glass ceramic is derived from cyclosiloxane monomers containing a vinyl group attached to silicon and/or a hydride-silicon group.

This application is a division, of application Ser. No. 07/464,470,filed Jan. 12, 1990.

PRIOR ART

The invention relates generally to composite laminates in which a matrixmaterial is reinforced with fibers. Laminates with a polymer matrix arewidely used for various purposes, but they are not generally applicablein situations where temperatures are expected to be above about 300° C.The present invention relates to ceramic fiber reinforced-glass matrixcomposites having application at temperatures which would destroyconventional polymeric materials.

Matrices having enhanced performance have been suggested for use withfibers having high strength at elevated temperatures. Examples of suchmatrix materials are the glass and glass ceramics (Prewo et al., CeramicBulletin, Vol. 65, No. 2, 1986).

In U.S. Ser. No. 002,049 a ceramic composition designated "black glass"is disclosed which has an empirical formula SiCxOy where x ranges from0.5 to about 2.0 and y ranges from about 0.5 to about 3.0, preferably xranges from 0.9 to 1.6 and y ranges from 0.7-1.8. Such a ceramicmaterial has a higher carbon content than prior art materials and isvery resistant to high temperatures--up to about 1400° C. This blackglass material is produced by reacting in the presence of ahydrosilylation catalyst a cyclosiloxane having a vinyl group with acyclosiloxane having a hydrogen group to form a polymer, which issubsequently pyrolyzed to black glass. The present invention involvesthe application of such black glass to reinforcing fibers to formlaminates very useful in high temperature applications.

In U.S. Pat. No. 4,460,638 a fiber-reinforced glass composite isdisclosed which employs high modulus fibers in a matrix of a pyrolyzedsilazane polymer. Another possible matrix material is the resin sol ofan organosilsesquioxane, as described in U.S. Pat. No. 4,460,639.However, such materials are hydrolyzed, and since they release alcoholsand contain excess water, they must be carefully dried to avoid fissuresin the curing process.

Another U.S. Pat. No. 4,460,640, disclosed related fiber reinforcedglass composites using organopolysiloxane resins of U.S. Pat. Nos.3,944,519 and 4,234,713 which employ crosslinking by the reaction of.tbd.SiH groups to CH₂ ═CHSi.tbd. groups. These later two patents havein common the use of organosilsesquioxanes having C₆ H₅ SiO_(3/2) units,which have been considered necessary by the patentees to achieve aflowable resin capable of forming a laminate. A disadvantage of such C₆H₅ SiO_(3/2) units is their tendency to produce free carbon whenpyrolyzed. The present invention requires no such C₆ H₅ SiO_(3/2) unitsand still provides a flowable resin, and does not produce easilyoxidized carbon.

Another disadvantage of the organopolysiloxanes used in the '640 patentis their sensitivity to water as indicated in the requirement that thesolvent used be essentially water-free. The resins contain silanolgroups and when these are hydrolyzed they form an infusible andinsoluble gel. The present invention requires no such silanol groups andis thus insensitive to the presence of water. In addition, theorganopolysiloxanes of the '640 patent may not have a long shelf lifewhile those of the present invention remain stable for extended periods.Still another disadvantage for the organopolysiloxanes disclosed in the'640 patent is that they require a partial curing step before pressingand final curing. This operation is difficult to carry out and mayprevent satisfactory lamination if the polymer is over cured. Thepresent invention can be carried out after coating the fibers andrequires no pre-curing step.

In co-pending U.S. patent application Ser. No. 07/426,820 composites ofrefractory fibers with a black glass matrix were disclosed. Suchcomposites have good physical properties but tend to exhibit brittlefracture with little evidence of fiber pullout. The composites reportedin U.S. Pat. Nos. 4,460,639 and 4,460,640 also exhibit brittle fracturewith a flexural strength of less than 308 MPa.

Ceramic matrix composites which combine whiskers, particulates, staples,or continuous fibers with ceramic matrix offer a potential to overcomethe catastrophic brittle failure inherent to monolithic ceramics. Amongthese reinforcement types, continuous fiber is the most effective meansknown for toughening ceramics. If brittle fracture is replaced by thegraceful fibrous fracture, ceramic composites may be used reliably as anengineering material for structural and other high performanceapplications.

The type of failure is to large extent determined by the nature of theinterface between the reinforcement fiber and the surrounding matrix. Inceramic composites, high toughness results when energy is absorbed asfibers pull out from the matrix as the composite cracks. Thus, a lowinterfacial stress or friction is needed to ensure fibrous fracture. Ifa strong interfacial bond exists, the crack will cut through the fiberwithout pulling out the fiber, resulting in a fracture behavior not muchdifferent from unreinforced monolithic ceramics. Our present inventionrelates to the use of a carbon interface in a silicon carboxide `black`glass matrix, producing a composite having a high strain-to-failure andexhibiting fibrous fracture.

SUMMARY OF THE INVENTION

An improved fiber reinforced glass composite of the invention comprises(a) at least one carbon-coated refractory fiber selected from the groupconsisting of boron, silicon carbide, graphite, silica, quartz, S-glass,E-glass, alumina, aluminosilicate, boron nitride, silicon nitride, boroncarbide, titanium boride, titanium carbide, zirconium oxide, andzirconia-toughened alumina and, (b) a carbon-containing black glassceramic composition having the empirical formula SiCxOy where x rangesfrom about 0.5 to about 2.0, preferably from 0.9 to 1.6, and y rangesfrom about 0.5 to about 3.0, preferably from 0.7 to 1.8.

In a preferred embodiment, the black glass ceramic composition (b) ofthe invention is the pyrolyzed reaction product of a polymer preparedfrom (1) a cyclosiloxane monomer having the formula ##STR1## where n isan integer from 3 to about 30, R is hydrogen, and R' is an alkene offrom 2 to about 20 carbon atoms in which one vinyl carbon atom isdirectly bonded to silicon or (2) two or more different cyclosiloxanemonomers having the formula of (1) where for at least one monomer R ishydrogen and R' is an alkyl group having from 1 to about 20 carbon atomsand for the other monomers R is an alkene from about 2 to about 20carbon atoms in which one vinyl carbon is directly bonded to silicon andR' is an alkyl group of from 1 to about 20 carbon atoms, saidpolymerization reaction taking place in the presence of an effectiveamount of hydrosilylation catalyst. The polymer product is pyrolyzed ina non-oxidizing atmosphere to a temperature in the range of about 800°C. to about 1400° C. to produce the black glass ceramic.

In another embodiment the invention comprises a method of preparing afiber reinforced glass composite wherein the cyclosiloxane reactionproduct described above is combined with carbon-coated refractory fiberswhich may be in the form of woven fabric or unidirectionally alignedfibers. Plies of the coated fibers may be laid-up to form a greenlaminate and thereafter pyrolyzed in a non-oxidizing atmosphere at atemperature between about 800° C. and about 1400° C., preferably about850° C., to form the black glass composite. The laminate may bereimpregnated with polymer solution and repyrolyzed in order to increasedensity. Alternatively, a resin transfer technique may be used in whichfibers, optionally having a carbon coating, are placed in a mold and theblack glass matrix precursor is added to fill the mold before curing toform a green molded product.

The refractory fibers are coated with a carbon layer about 0.01 μm to 5μm thick prior to fabrication and pyrolysis of the cyclosiloxanes toform the black glass matrix. Preferred methods of forming such carboncoatings are chemical vapor deposition, solution coating, and pyrolysisof organic polymers such as carbon pitch and phenolics.

These uniaxial silicon carbide fiber reinforced black glass compositesshow flexural strength greater than about 750 MPa at room temperatureand fibrous, graceful fracture at temperatures below about 600° C. Afive-fold increase in flexural strength and a six-fold increase instrain at maximum stress has been obtained as compared with black glasscomposites without a carbon interfacial coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIG. is a graph comparing the flexural strengths of compositesof uncoated and coated Nicalon® fibers in black glass matrices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Black Glass Ceramic

The black glass ceramic used as the matrix has an empirical formulaSiCxOy wherein x ranges from about 0.5 to about 2.0, preferably 0.9-1.6,and y ranges from about 0.5 to about 3.0, preferably 0.7-1.8, wherebythe carbon content ranges from about 10% to about 40% by weight. Theblack glass ceramic is the product of the pyrolysis in a non-oxidizingatmosphere at temperatures between about 800° C. and about 1400° C. of apolymer made from certain siloxane monomers.

The polymer precursor of the black glass ceramic may be prepared bysubjecting a mixture containing cyclosiloxanes of from 3 to 30 siliconatoms to a temperature in the range of from about 10° C. to about 300°C. in the presence of 1-200 wt. ppm of a platinum hydrosilylationcatalyst for a time in the range of from about 1 minute to about 600minutes. When the polymer is placed in a nonoxidizing atmosphere, suchas nitrogen, and pyrolyzed at a temperature in the range from about 800°C. to about 1400° C. for a time in the range of from about 1 hour toabout 300 hours, black glass results. The polymer formation takesadvantage of the fact that a silicon-hydride will react with asilicon-vinyl group to form a silicon-carbon-carbon-silicon bondedchain, thereby forming a network polymer. For this reason, each monomercyclosiloxane must contain either a silicon-hydride bond or asilicon-vinyl bond or both. A silicon-hydride bond refers to a siliconatom bonded directly to a hydrogen atom and a silicon-vinyl bond refersto a silicon atom bonded directly to an alkene carbon, i.e., it isconnected to another carbon atom by a double bond.

The polymer precursor for the black glass ceramic may be definedgenerally as the reaction product of (1) a cyclosiloxane monomer havingthe formula ##STR2## where n is an integer from 3 to 30, R is hydrogen,and R' is an alkene of from 2 to 20 carbon atoms in which one vinylcarbon atom is directly bonded to silicon or (2) two or more differentcyclosiloxane monomers having the formula of (1) where for at least onemonomer R is hydrogen and R' is an alkyl group having from 1 to 20carbon atoms and for the other monomers R is an alkene from about 2 to20 carbon atoms in which one vinyl carbon is directly bonded to siliconand R' is an alkyl group of from 1 to 20 carbon atoms, said reactiontaking place in the presence of an effective amount of hydrosilylationcatalyst.

The black glass ceramic may be prepared from a cyclosiloxane polymerprecursor wherein both the requisite silicon-hydride bond and thesilicon-vinyl bond are present in one molecule, for example,1,3,5,7-tetravinyl-1,3,5,7-tetrahydrocyclotetrasiloxane. Alternatively,two or more cyclosiloxane monomers may be polymerized. Such monomerswould contain either a silicon hydride bond or a silicon-vinyl bond andthe ratio of the two types of bonds should be about 1:1, more broadlyabout 1:9 to 9:1.

Examples of such cyclosiloxanes include, but are not limited to:

1,3,5,7-tetramethyltetrahydrocyclotetrasiloxane,

1,3,5,7-tetravinyltetrahydrocyclotetrasiloxane,

1,3,5,7-tetravinyltetraethylcyclotetrasiloxane,

1,3,5,7-tetravinyltetramethylcyclotetrasiloxane,

1,3,5-trimethyltrivinylcyclotrisiloxane,

1,3,5-trivinyltrihydrocyclotrisiloxane,

1,3,5-trimethyltrihydrocyclotrisiloxane,

1,3,5,7,9-pentavinylpentahydrocyclopentasiloxane,

1,3,5,7,9-pentavinylpentamethylcyclopentasiloxane,

1,1,3,3,5,5,7,7-octavinylcyclotetrasiloxane,

1,1,3,3,5,5,7,7-octahydrocyclotetrasiloxane,

1,3,5,7,9,11-hexavinylhexamethylcyclohexasiloxane,

1,3,5,7,9,11-hexamethylhexahydrocyclohexasiloxane,

1,3,5,7,9,11,13,15,17,19-decavinyldecahydrocyclodecasiloxane,

1,3,5,7,9,11,13,15,17,19,21,23,25,27,29-pentadecavinylpentadecahydrocyclopentadecasiloxane

1,3,5,7-tetrapropenyltetrahydrocyclotetrasiloxane,

1,3,5,7-tetrapentenyltetrapentylcyclotetrasiloxane and

1,3,5,7,9-pentadecenylpentapropylcyclopentasiloxane.

It will be understood by those skilled in the art that while thesiloxane monomers may be pure species, it will be frequently desirableto use mixtures of such monomers, in which a single species ispredominant. Mixtures in which the tetramers predominate have been foundparticularly useful.

While the reaction works best if platinum is the hydrosilylationcatalyst, other catalysts such as cobalt and manganese carbonyl willperform adequately. The catalyst can be dispersed as a solid or can beused as a solution when added to the cyclosiloxane monomer. Withplatinum, about 1 to 200 wt. ppm, preferably 1 to 30 wt. ppm will beemployed as the catalyst.

Black glass precursor polymer may be prepared from either bulk orsolution polymerization. In bulk polymerization, neat monomer liquid,i.e., without the presence of solvents reacts to form oligomers or highmolecular weight polymers. In bulk polymerization, a solid gel can beformed without entrapping solvent. It is particularly useful forimpregnating porous composites to increase density. Solutionpolymerization refers to polymerizing monomers in the presence of anunreactive solvent. Resin used in impregnating fibers to form prepreg inour invention preferably is prepared by solution polymerization. Theadvantage of solution polymerization is the ease of controlling resincharacteristics. It is possible but very difficult to produce B-stageresin suitable for prepregs with consistent characteristics by bulkpolymerization. In the present invention, soluble resin with thedesirable viscosity, tackiness, and flowability suitable for prepreggingand laminating can be obtained consistently using solutionpolymerization process. The production of easily handleable andconsistent resin is very critical in composite fabrication.

Fibers

Reinforcing fibers useful in the composites of the invention arerefractory fibers which are of interest for applications where superiorphysical properties are needed. They include such materials as boron,silicon carbide, graphite, silica, quartz, S-glass, E-glass, alumina,aluminosilicates, boron nitride, silicon nitride, boron carbide,titanium boride, titanium carbide, zirconium oxide, andzirconia-toughened alumina.

The fibers may have various sizes and forms. They may be monofilamentsfrom 1 μm to 200 μm diameter or tows of 200 to 2000 filaments. When usedin composites of the invention they may be woven into fabrics, pressedinto mats, or unidirectionally aligned with the fibers oriented asdesired to obtain the needed physical properties.

An important factor in the performance of the black glass composites isthe bond between the fibers and the black glass matrix. Consequently,where improved tensile strength is desired, the fibers are provided witha carbon coating which reduces the bonding between the fibers and theblack glass matrix. The surface sizings found on fibers as received orproduced may be removed by solvent washing or heat treatment and thecarbon coating applied. Various methods may be used, including chemicalvapor deposition, solution coating, and pyrolysis of organic polymerssuch as carbon pitch and phenolics. One preferred technique is chemicalvapor deposition using decomposition of methane or other hydrocarbons.Another method is pyrolysis of an organic polymer coating such asphenolformaldehyde polymers cross-linked with such agents as themonohydrate or sodium salt of paratoluenesulfonic acid. Still anothermethod uses toluene-soluble and toluene-insoluble carbon pitch to coatthe fibers. After pyrolysis, a uniform carbon coating is present.Multiple applications may be used to increase the coating thickness.

Processing

As previously discussed, the black glass precursor is a polymer. It maybe shaped into fibers and combined with reinforcing fibers or the blackglass precursor may be used in solution for coating or impregnatingreinforcing fibers. Various methods will suggest themselves to thoseskilled in the art for combining the black glass precursor withcarbon-coated reinforcing fibers. It would, for example, be feasible tocombine fibers of the polymer with fibers of the reinforcing materialand then to coat the resulting fabric or mat. Alternatively, thereinforcing fibers could be coated with a solution of the polymer andthen formed into the desired shape. Coating could be done by dipping,spraying, brushing, or the like. In still another embodiment, the resintransfer technique can be employed in which the reinforcing fibers areplaced in a mold and then the black glass precursor is added to fill themold before curing to form a green molded product.

In one method, a continuous fiber is coated with a solution of the blackglass precursor polymer and then wound on a rotating drum covered with arelease film which is easily separated from the coated fibers. Aftersufficient fiber has been built up on the drum, the process is stoppedand the unidirectional fiber mat removed from the drum and dried. Theresulting mat (i.e., "prepreg") then may be cut and laminated into thedesired shapes.

In a second method, a woven or pressed fabric of the reinforcing fibersis coated with a solution of the black glass precursor polymer and thendried, after which it may be formed into the desired shapes byprocedures which are familiar to those skilled in the art of fabricatingstructures with the prepreg sheets. For example, layers of prepregsheets may be laid together and pressed into the needed shape. Theorientation of the fibers may be chosen to strengthen the composite partin the principal load-bearing directions.

A third method for fabricating the polymer composite is by resintransfer molding. In resin transfer molding a mold with the requiredshape is filled with the desired reinforcement material. Thereinforcement could be a perform having a 3-dimensional weave of fibers,a lay-up of fabric plies, a non-woven mat of chopped staple or bundledtows, or assemblies of whiskers, and such others as are familiar tothose skilled in the art. The reinforcement material would be coatedwith the carbon to insure a weak bond between matrix and reinforcementin the final composite where improved tensile strength is desired.Carbon coating may be omitted where the end use does not require hightensile strength.

The filled mold is injected, preferably under vacuum, with the neatmonomer solution with an appropriate amount of catalyst. The relativeamounts of vinyl- and hydro-monomers will be adjusted to obtain thedesired carbon level in the pyrolyzed matrix. The low viscosity (<50centipoise) of the neat monomer solution is exceptionally well suitedfor resin impregnation of thick wall and complex shape components.

The filled mold is then heated to about 30° C.-150° C. for about 1/2-30hours as required to cure the monomer solutions to a fully polymerizedstate. The specific cure cycle is tailored for the geometry and desiredstate of cure. For example, thicker wall sections require slower curesto prevent uneven curing and exothermic heat build-up. The cure cycle istailored through control of the amount of catalyst added and thetime-temperature cycle. External pressure may be used during the heatingcycle as desired.

When the component is fully cured, it is removed from the mold. In thiscondition it is equivalent in state to the composite made by laminationand autoclaving of prepreg plies. Further processing consists of theequivalent pyrolysis and impregnation cycles to be described for thelaminated components.

Solvents for the black glass precursor polymers include aromatichydrocarbons, such as toluene, benzene, and xylene, and ethers, such astetrahydrofuran, etc. Concentration of the prepregging solution may varyfrom about 10% to about 70% of resin by weight. Precursor polymer usedin impregnating the fibers is usually prepared from solutionpolymerization of the respective monomers.

Since the precursor polymers do not contain any hydrolyzable functionalgroups, such as silanol, chlorosilane, or alkoxysilane, the precursorpolymer is not water sensitive. No particular precaution is needed toexclude water from the solvent or to control relative humidity duringprocessing.

Our resin ages very slowly when stored at or below room temperatures asis evident from their shelf life of more than three months at thesetemperatures. The resin is stable both in the solution or in theprepreg. Prepregs stored in a refrigerator for three months have beenused to make laminates without any difficulty. Also, resin solutionsstored for months have been used for making prepregs successfully.

Large and complex shape components can be fabricated from laminatingprepregs. One method is hand lay-up which involves placing theresin-impregnated prepregs manually in an open mold. Several plies ofprepregs cut to the desired shape are laid-up to achieve the requiredthickness of the component. Fiber orientation can be tailored to givemaximum strength in the preferred direction. Fibers can be orientedunidirectionally [0], at 90° angles [0/90], at 45° angles [0/45 or45/90], and in other combinations as desired. The laid-up plies are thenbonded by vacuum compaction before autoclave curing. Another fabricationmethod is tape laying which uses pre-impregnated ribbons in formingcomposites. Our resins can be controlled to provide the desiredtackiness and viscosity in the prepreg for the lay-up procedures.

After the initial forming, the composites may be consolidated and curedby heating to temperatures up to about 250° C. under pressure. In onemethod, the composited prepreg is placed in a bag, which is thenevacuated and the outside of the bag placed under a pressure sufficientto bond the layered prepreg, say up to about 1482 kPa. The resin canflow into and fill up any voids between the fibers, forming a void-freegreen laminate. The resulting polymer-fiber composite is dense and isready for conversion of the polymer to black glass ceramic. If anexcessively cured prepreg is used, as is possible with the method ofU.S. Pat. No. 4,460,640, there will be no adhesion between the plies andno flow of resin material and no bonding will occur.

Heating the composite to temperatures from about 800° C. up to about1400° C. in an inert atmosphere (pyrolysis) converts the polymer into ablack glass ceramic containing essentially only carbon, silicon, andoxygen. It is characteristic of the black glass prepared by pyrolyzingthe cyclosiloxanes described above that the resulting black glass has alarge carbon content, but is able to withstand exposure to temperaturesup to about 1400° C. in air without oxidizing to a significant degree.Pyrolysis is usually carried out with a heating to the maximumtemperature selected, holding at that temperature for a period of timedetermined by the size of the structure, and then cooling to roomtemperature. Little bulk shrinkage is observed for the black glasscomposites and the resulting structure typically has about 70-80% of itstheoretical density.

Conversion of the polymer to black glass takes place between 430° C. and950° C. Three major pyrolysis steps were identified by thermogravimetricanalysis at 430° C.-700° C., 680° C.-800° C. and 780° C.-950° C. Theyield of the polymer-glass conversion up to 800° C. is about 83%; thethird pyrolysis mechanism occurring between 780° C. and 950° C.contributed a final 2.5% weight loss to the final product.

Since the pyrolyzed composite structure still retains voids, thestructure may be increased in density by impregnating with a neatmonomer liquid or solution of the black glass precursor polymer. Thesolution is then gelled by heating to about 50° C.-120° C. for asufficient period of time. Following gelation, the polymer is pyrolyzedas described above. Repeating these steps, it is feasible to increasethe density up to about 95% of the theoretical.

The above procedures will be illustrated in more detail in the examplesof co-pending application U.S. Ser. No. 07/426,820. The examples belowillustrate the advantages obtained by applying a carbon coating to therefractory fibers prior to contacting them with the black glassprecursors and the formation of reinforced black glass articles by resintransfer molding.

EXAMPLE 1 Polymer Precursor Preparation

The cyclosiloxane having silicon-vinyl bond waspoly(vinylmethylcyclosiloxane) (ViSi). The cyclosiloxane with asilicon-hydride bond was poly(methylhydrocyclosiloxane) (HSi). Bothcyclosiloxanes were mixtures of oligomers, about 85% by weight being thecyclotetramer with the remainder being principally the cyclopentamer andcyclohexamer. A volume ratio of 59 ViSi/41 HSi was mixed with 22 wt. ppmof platinum as a platinum-cyclovinylmethylsiloxane complex in toluene togive a 10 vol. percent solution of the cyclosiloxane monomers. Thesolution was heated to reflux conditions (about 110° C.) and refluxedfor about 2 hours. Then, the solution was concentrated in a rotaryevaporator at 50° C. to a 25-35% concentration suitable for use inprepregging. The resin produced was poly(methylmethylenecyclosiloxane)(PMMCS). It was hard and dry at room temperature, but it was flowable attemperatures of about 70° C. or higher and thus suitable for use as a Bstage resin.

EXAMPLE 2 Preparation of Test Specimens

A 26 wt. % poly(methylmethylenecyclosiloxane) (PMMCS) solution intoluene was used for making a prepreg. The viscosity of the resinsolution was 2.98 centipoise. Carbon-coated continuous ceramic gradeNicalon® tow containing 500 monofilaments (a silicon carbide fibersupplied by Dow-Corning) was impregnated with the PMMCS resin by passingthe tow through the resin solution. The carbon coating had been appliedusing chemical vapor deposition and was 0.1 to 0.3 μm thick. The sizingfor the tow was poly (vinyl alcohol) (PVA). The impregnated tow wasformed into a prepreg by laying up the tow on a rotating drum. Theprepreg contained 25.1% by weight of PMMCS and 74.9% by weight fiber.The areal weight, which is defined as the weight of fiber per unit areain the prepreg, was 308 gm/m².

6"×3.75" plies were cut from the prepreg. Eight piles were laid-upunidirectionally to form a laminate. This [0]₈ laminate was consolidatedusing the following procedure:

1. compacting under vacuum at room temperature for 1/2 hour,

2. debulking at 55° C.-65° C. for 1/2 hour under vacuum,

3. heating to 150° C. at 100 psi (689.7 kPa gauge) nitrogen pressureover two hours, and

4. cooling to room temperature.

The resin flowed and solidified during the autoclave curing. Loss of theresin through bleeding was estimated to be less than 2% with respect tothe total weight of the laminate.

The consolidated green laminate was then machine cut into 0.26"×2.00"(6.6 mm×50.8 mm) test bars with average thickness of 0.086" (2.18 mm).The green test bars were then pyrolyzed in flowing nitrogen(flowrate=ca. 700 cubic cm per minute) to convert the PMMCS into blackglass matrix composites using the following heating program:

1. heat to 850° C. in 8 hours,

2. hold at 850° C. for 1 hour, and

3. cool to room temperature over 8 hours.

The density of the as-pyrolyzed test bar was 1.7 gm/cc with a char yieldof 96.8%. The test bars were then infiltrated with the neat monomerliquid without solvent. After gelling the sol at 50°-70° C., theinfiltrated bars were then pyrolyzed using the same program as describedabove. A total of six impregnations were used to increase the density ofthe composite to about 2.13 gm/cc. Bars impregnated six times contained60% Nicalon® fiber by volume. Open porosity was estimated to be about7.1%.

EXAMPLE 3 Testing for Flexural Strength

4-point bend tests were performed on the carbon-coated Nicalon®reinforced black glass bars prepared in Example 2 using an Instrontester. The outer span of the fixture was 1.5 inches (38.1 mm) with 0.75inches (19 mm) inner span, giving a span-to-depth ratio of 17.5.Flexural strengths and densities for various levels of impregnation aresummarized below.

    ______________________________________                                                   # of      Strength    density                                      Impregnations                                                                            Samples   MPa (sdev)  gm/cc (sdev)                                 ______________________________________                                        1          3         35.85   (9)   1.92 (.24)                                 2          3         86.87   (8.3) 1.91 (.01)                                 3          4         123.4   (25.5)                                                                              1.97 (0.03)                                6          4         631.6   (117) 2.13 (0.05)                                ______________________________________                                    

Samples tested after 1, 2, and 3 impregnations showed deformation at theload points but did not break. The maximum load was used to calculatethe strength. Samples impregnated six times achieved densities around89.5% of theoretical. Fibrous fracture was observed for samplesimpregnated six times, which exhibited about 0.6% strain at maximumstress. For comparison, similar black glass composites prepared withuncoated Nicalon® fibers had flexural strengths of 151.2 MPa, densitiesof 2.19 g/cc, strain at maximum stress of 0.14%, and exhibit brittlefracture. This example demonstrates the importance of carbon coatings onthe increase in strength and toughness of the black glass matrixcomposites.

EXAMPLE 4

A consolidated green laminate was prepared using the procedure describedin Example 2. Test bars that were 5.5 inches long by 0.4 inches wide by0.07 inches thick (139.7 mm×10.16 mm×1.78 mm) were cut from the laminateand pyrolyzed. After five impregnation and pyrolysis cycles, thesespecimens had a density of 2.13 g/cc. These test bars were tested infour point bending mode using a lower span of 4.5 inches (114.3 mm) andan upper span of 2.25 inches (57.2 mm). Mean bend strength was 768.8 MPawith a strain at maximum stress of 0.9%. These samples exhibited fibrousfracture. A representative stress-strain curve for these test bars isshown in the Figure.

Nicalon® fibers without a carbon coating were used to prepare SiC fiberreinforced black glass composites using a procedure similar to thatdescribed in Example 2. Test bars that were 4 inches by 0.5 inches by0.065 inches (101.5 mm×12.7 mm×1.65 mm) were impregnated and pyrolyzedfive times to a density of 2.13 g/cc. These bars were tested in fourpoint bending mode using lower spans of 2 and 3 inches (50.8 mm and 76.2mm) with upper spans of 1 and 1.5 inches (25.4 mm and 38.1 mm),respectively. The mean bend strength was 144.8 MPa with a strain atmaximum stress of 0.14%. All samples exhibited brittle failure. Arepresentative stress-strain curve for this brittle material is alsoshown in the Figure. This example demonstrates the importance of carboncoatings on the increase in strength and strain at maximum stress forthe black glass matrix composites.

EXAMPLE 5

A consolidated green laminate was formed using the procedure in Example2. Test bars 7.5 inches long by 0.4 inches wide (190.5 mm×10.2 mm) werecut from the panel and pyrolyzed. After 5 impregnation and pyrolysiscycles, a strain gage was mounted on one of the surfaces. This test barwas tested in three point bending geometry with a six inch (152.4 mm)span. The strain gage was on the tensile surface of the bar. Maximumflexural stress was observed at 737.7 MPa with a strain at maximumstress of 0.9%.

EXAMPLE 6

Black glass matrix composites with carbon-coated Nicalon® were alsoimpregnated with the PMMCS resin diluted with toluene. A solution havingabout 50 wt. % resin was used for infiltration. The amount of matrixmaterial incorporated into the composite in the PMMCS solution processwould be less than when the neat monomer is used for the sameimpregnation cycles. Therefore, the solution impregnated composites havelower densities than their corresponding neat liquid impregnatedsamples. Strengths and densities are summarized as below.

    ______________________________________                                                   # of      Strength    density                                      Impregnations                                                                            Samples   MPa (sdev)  gm/cc (sdev)                                 ______________________________________                                        2          4         73.1    (8.3) 1.91 (0.06)                                4          3         187.5   (22.8)                                                                              2.02 (0.05)                                5          4         261.3   (64.8)                                                                              2.03 (0.06)                                ______________________________________                                    

These bars deformed under the load points but did not fracture.

EXAMPLE 7

A set of carbon-coated Nicalon® test bars were prepared following thesame procedure as described in Example 2. The total weight of the greentest bars was 45.0193 gm. After five impregnations, the final totalweight of the test bars was 54.8929 gm, an increase of 21.3% withrespect to the green state. Fiber content in the infiltrated samples was59.2 vol. %, or 62.8 wt. %. The density of these samples was 2.17 gm/cc,about 90.5% theoretical.

Room temperature 3-point bend tests on 9 samples using a span/depthratio of 23.5 gave an average strength of 765.3 MPa (48.3 sdev). Thespecimens tested in 3-point bend failed in tensile mode and exhibitedfibrous failure.

Five bars were heat-treated in stagnant air at 800° C. for 16 hours.After the oxidation, the samples lost about 1% in weight. A roomtemperature 4-point bend test showed brittle failure with bend strengthof 135.1 MPa (standard deviation=15.2 MPa). These results indicate lossof carbon interface and oxidation of the silicon carbide fibers,resulting in strong bonding between fiber and matrix.

Black glass-carbon coated Nicalon® composite test bars aged at 315° C.,350° C., 400° C., and 450° C. for 60 hours in stagnant air were flexuretested at room temperature and elevated temperatures.

    ______________________________________                                        Aging         Test       Flexural                                             Temperature   Temperature                                                                              Strength (MPa)                                       ______________________________________                                        none          Room Temp. 806.7                                                315° C.                                                                              Room Temp. 806.7                                                350° C.                                                                              Room Temp. 455.1                                                350° C.                                                                              350° C.                                                                           489.5                                                400° C.                                                                              Room Temp. 289.6                                                400° C.                                                                              400° C.                                                                           337.8                                                450° C.                                                                              Room Temp. 324                                                  ______________________________________                                    

Samples aged below 450° C. exhibited fibrous fracture. Full strength isretained for heat treatment in air below 315° C., whereas lowerstrengths at higher temperatures indicate the carbon layer has beendegraded. Although the overall strengths are degraded by aging, samplestested at the aging temperature have strengths identical withinexperimental error to the room-temperature strengths.

EXAMPLE 8 Test Specimens by Resin-Transfer Molding

Nicalon® woven fabric plies cut to shape, with or without carboncoating, are stacked or placed into a 152.4 mm×152.4 mm×2.54 mm mold anda 45 wt. % solution of PMMCS precursors is introduced to fill the mold.The solution is 61 volume percent ViSi and 39 volume percent SiH and ismixed with about 10 wt. ppm of the platinum complex used in Example 1.By heating to about 55° C. over 5 hours the solution is gelled to form agreen composite, which is removed from the mold, cut into test bars152.4 mm long by 10.2 mm wide, and pyrolyzed at temperatures up to 850°C. as previously described. Further improvement in density is obtainedby subsequent impregnations with neat monomer liquid as previouslydescribed. The samples are then available for testing.

EXAMPLE 9 Resin-Transfer Molding

A Nextel® 440 (from 3-M) Techniweave was used for resin transfermolding. A 63.5 mm×50.8 mm×5.1 mm Techniweave was cut and weighed to be18.40 gm. Black glass precursor liquid consisting of 61%vinylmethylcyclosiloxane and 39% hydromethylcyclosiloxane was mixed withabout 10 ppm soluble Pt catalyst complex as in Example 1. The viscosityof the precursor liquid is about 1 centipoise. The weave was placed in ajar and vacuum infiltrated with the liquid. The infiltrated weave wasgelled by heating at 55° C. for 5 hours, forming a consolidated greencomposite. The green composite was removed from the jar and pyrolyzed to850° C. in flowing nitrogen to effect black glass conversion. Theas-fabricated composite was reimpregnated with the same precursor liquidto increase the density. The bulk density of the Nextel® 440 Techniweavepreform was about 1.10 gm/cc. After a total of 5 impregnations, thedensity was increased to 2.07 gm/cc with 9.5% open porosity.

EXAMPLE 10 RESIN TRANSFER MOLDING

6.35 mm long alumina FP staple was packed into a 57.2 mm diametercylindrical bronze cup, using a uniaxial press. The amount of FP staplewas 142.1 gm. The packed staple block was vacuum infiltrated with asolution containing 61 vol. % ViSi and 39 wt. % SiH and about 10 ppmsoluble Pt catalyst samples. The monomers are gelled at 55° C. for 12hours and further hardened at 110° C. for 2 hours. The consolidatedblock was then removed from the cup and pyrolyzed in flowing nitrogen.The heating procedure included heating to 400° C. in 10 hours, from 400°C. to 500° C. in 15 hours, from 500° C. to 850° C. in 25 hours, andcooling to room temperature in 12 hours. The as-pyrolyzed block weighed192.1 gm and was hard and rigid. No macro-cracks were observed. Theblock was cut in half for inspection of interior morphology. Uniformdistribution of staple and matrix material across the interiorcross-section was found. The density of the block was estimated to beabout 1.95 gm/cc. After a total of 6 impregnation/pyrolysis cycles, thedensity of the sample was 2.50 gm/cc with open porosity of 13%.

We claim:
 1. A method of preparing fiber reinforced glass compositescomprising:(a) reacting (1) a cyclosiloxane monomer having the formula##STR3## where n is an integer from 3 to 30, R is hydrogen, and R' is analkene of from 2 to 20 carbon atoms in which one vinyl carbon atom isdirectly bonded to silicon or (2) two or more different cyclosiloxanemonomers having the formula of (1) where for at least one monomer R ishydrogen and R' is an alkyl group having from 1 to 20 carbon atoms andfor the other monomers R is an alkene from 2 to 20 carbon atoms in whichone vinyl carbon is directly bonded to silicon and R' is an alkyl groupof from 1 to 20 carbon atoms, said reaction taking place in the presenceof an effective amount of hydrosilylation catalyst; (b) applying thereaction product of (a) to at least one refractory fiber having a carboncoating about 0.01 μm to 5 μm thick and selected from the groupconsisting of boron, silicon carbide, graphite, silica, quartz, S-glass,E-glass, alumina, aluminosilicate, boron nitride, silicon nitride, boroncarbide, titanium boride, titanium carbide, zirconium oxide andzirconiatoughened alumina to form a prepreg; (c) laying-up plies of theprepreg of (b) to form a green structure; (d) curing the green structureof (c) at a temperature not greater than 250° C.; (e) pyrolyzing thecured structure of (d) at a temperature of about 800° C. to about 1400°C. in non-oxidizing atmosphere; (f) recovering the pyrolyzed product of(e) as the fiber reinforced glass composite; (g) impregnating thepyrolyzed product of (f) with the reaction product of (a); (h)pyrolyzing the impregnated product of (g) at 800° C.-1400° C.; (i)repeating steps (g) and (h) to achieve the desired density.
 2. Themethod of claim 1 wherein the pyrolysis of (e) is carried out at atemperature of about 850° C.
 3. The method of claim 1 wherein saidcarbon coating is deposited by chemical vapor deposition.
 4. The methodof claim 1 wherein said carbon coating is a pyrolyzed organic polymer.5. The method of claim 4 wherein said pyrolyzed organic polymer is apyrolyzed carbon pitch.
 6. The method of claim 4 wherein said pyrolyzedorganic polymer is a pyrolyzed phenol-formaldehyde polymer.
 7. Themethod of claim 1 wherein said refractory fibers of (b) are in the formof a woven fabric.
 8. The method of claim 1 wherein said refractoryfibers of (b) are unidirectional and continuous.
 9. A method ofpreparing fiber reinforced glass composites comprising:(a) placing intoa mold at least one refractory fiber, optionally having a carbon coatingabout 0.01 μm to 5 μm thick and selected from the group consisting ofboron, silicon carbide, graphite, silica, quartz, S-glass, E-glass,alumina, aluminosilicate, boron nitride, silicon nitride, boron carbide,titanium boride, titanium carbide, zirconium oxide, andzirconia-toughened alumina; (b) filling the mold of (a) with (1) acyclosiloxane monomer having the formula ##STR4## where n is an integerfrom 3 to 30, R is hydrogen, and R' is an alkene of from 2 to 20 carbonatoms in which one vinyl carbon atom is directly bonded to silicon or(2) two or more different cyclosiloxane monomers having the formula of(1) where for at least one monomer R is hydrogen and R' is an alkylgroup having from 1 to 20 carbon atoms and for the other monomers R isan alkene from 2 to 20 carbon atoms in which one vinyl carbon isdirectly bonded to silicon and R' is an alkyl group of from 1 to 20carbon atoms, and an effective amount of hydrosilylation catalyst; (c)reacting the monomers of (b) at a temperature of about 30° C. to 150° C.to form a green composite; (d) pyrolyzing the green composite of (c) ata temperature of about 800° C. to about 1400° C. in non-oxidizingatmosphere; (e) recovering the pyrolyzed product of (d) as the fiberreinforced glass composite; (f) impregnating the pyrolyzed product of(e) with the reaction product of the monomers of (b); (g) pyrolyzing theimpregnated product of (f) at 800° C.-1400° C.; (h) repeating steps (f)and (g) to achieve the desired density.
 10. The method of claim 9wherein the pyrolysis of (d) is carried out at a temperature of about850° C.
 11. The method of claim 9 wherein said carbon coating isdeposited by chemical vapor deposition.
 12. The method of claim 9wherein said carbon coating is a pyrolyzed organic polymer.
 13. Themethod of claim 12 wherein said pyrolyzed organic polymer is pyrolyzedcarbon pitch.
 14. The method of claim 12 wherein said pyrolyzed organicpolymer is a pyrolyzed phenol-formaldehyde polymer.
 15. The method ofclaim 9 wherein said refractory fiber is a preform having athree-dimensional weave.
 16. The method of claim 9 wherein saidrefractory fiber is a lay-up of fabric plies.
 17. The method of claim 9wherein said refractory fiber is a staple fiber.
 18. The method of claim9 wherein said refractory fiber is an assembly of whiskers.